Genetics: Part 1

The human here. As Tia mentioned yesterday, all this week, I’ve been allowed to post on her blog to address some things that I hope will help others if they ever find themselves in a position where they have a mum cat to look out for. I really hope this week’s posts help even one person.






So, your cat is pregnant. One of the most common questions people ask is what colour the kittens will be. This all comes down to genetics, and they are complex enough that I’m going to split them into two posts. I’ll start with the basics today.

What is a gene?

In its simplest form, a gene is a part of your DNA. It is a chain of different molecules which code for the production of a protein in the body. These proteins and coding strands are responsible for many things from hair colour to height, skin tone and even weight to some extent (no, that can’t be used as a valid reason to go eat chocolate!). In short, genes are the things that make you look the way you do, and often, have the health problems that you experience. They are passed to you from your mother and your father. In this post, I’ll be looking at genes specifically linked to coat colour and pattern, as these are the best understood so far in the cat world.

How do we get genes?

At the point of fertilization, a sperm from the father and an egg from the mother join. Each of these cells carries genetic, DNA information which binds together when the cells join. DNA, and therefore genes, are housed on things called chromosomes. Most are x-shaped, one half being formed from the mother’s DNA and the other from the father’s. Genes therefore have a pair partner on the opposite strand of the chromosome. However, whereas a female possesses all X shaped chromosomes, a male has one in each cell that is a Y shape. It is missing a leg. This means that genes on the opposite corresponding leg have no pair partner. Cat coat colour genes exist on this chromosome, which explains why girls only are torties. Boys can only express one colour at a time due to not having a pair partner in the DNA for the colour gene. Those male torties that do exist are sterile. This is because they not only have an x-y chromosome as normal males do, but they have an XXY set-up, meaning they have an extra, unwanted chromosome. This has the added effect of making them sterile.

Dominant and Recessive/dilute

Genes come in two types. These are called dominant and recessive. Dominant genes do what their name suggests; they dominate. Just one dominant gene will mean that a cat exhibits the trait linked to that gene. Recessives cannot manifest alone. There needs to be two recessives in order for the physical trait to be noticed. To understand genes, you need to remember that they always come in pairs, one from the mother and one from the father. When it comes to coat colour, the recessive gene commonly seen is called a dilute gene. This is because it dilutes the true colour of the coat to form something else.





A person can be said to be a carrier of a recessive, but not a dominant. Why? Because carriers possess the gene, but do not show the physical trait. Remember, if a person, or a cat, has a dominant gene, they will show that trait even if they only have one dominant. So, if they have a dominant, they cannot carry the gene because they show the trait. The same is true if they possess two recessives. However, if they have only one recessive, then they will possess the gene but not the trait, so can be called a carrier. This will be important when we come on to predicting colour possibilities in cats.





Speaking of cats, let’s get to colour. Believe it or not, there are only two true colours in cats. Despite the profusion of different coloured coats and combinations out there, they all come from only two colours. Those are black and red. Every coat colour’s genetics can be traced back to either black or red. But how? That’s what I’m going to try and explain. And that’s when it gets complicated.





Dilutes, the recessives remember, are very important in coat colour in cats, as they allow deviation from the blacks and reds. Black and red are dominant, but the dilutes, even though they are recessive, are able to work in tandem with this dominant to alter it. If a cat has the dominant gene for black, but also possesses two dilute genes, then instead of black, their coat will turn blue. Likewise, if another cat has the dominant for red, two dilutes will work with the red gene to produce a cream coat.





But you’ve seen cats that show black and red together, haven’t you? Or cats who show blue and cream? Well, that’s very common. These cats are called torties and blue torties/blue-creams. Most of them are female, but a very small percentage can be male. However, because colour genes are sex linked, any tortie male is sterile and can never reproduce. Females exhibit black and red or the dilute forms of them due to having two x chromosomes. So although the two are dominant, they can both exist in harmony given their position on chromosomes. If a boy had two x chromosomes, he could do the same thing.





So, by the logic we’ve already explored, you couldn’t have a cat with red and cream in its coat. Why? Because it takes two dilute genes to make cream, and if you have two dilutes, they cancel out the red. Similarly, you couldn’t have a cat with black and blue. The same is true when you look at red and blue or cream and black. I hope you understand why. Remember, two recessives/dilutes code for cream and blue, so when they’re there, they cancel out the red and black. Blue-cream exists together because both are dilute colours. Red and black exist together because both are dominant colours.

Chocolate

Just to complicate things further, there’s another way that colours can vary. Well, there’s more than one, but let’s keep it simple. Chocolate is rare, and is an alteration of the black dominant gene.





Ok, I lied when I said we’d keep it simple, because chocolate isn’t. It’s the interaction between a dominant and a recessive which can also be altered by a dilute. Confused? I was for a long time. Basically, chocolate is a recessive gene. That means, as with the dilute, that two copies of chocolate must exist before the cat can have a chocolate colour visible on its coat. One copy alone makes the cat a chocolate carrier. However, here’s where things get extra tricky. Chocolate, although it is a recessive which will modify only the black dominant gene, can itself be modified by that old friend, the dilute. If a cat has two copies of chocolate and two dilutes, then it makes lilac. Obviously, if a cat only has one dilute then chocolate still wins. If it has only one chocolate, then the dilute will act on the dominant coat colour, red or black.





Ready for more complication? Chocolate cannot impact or modify the red gene, so if you have a red boy or girl, they can have two copies of the chocolate gene and not show it. They can also have two copies of the dilute and still not show chocolate or lilac, although in this case, the base colour of the coat would be cream, not red (remember, red plus two dilutes =cream).

White

Although this isn’t exactly a colour, it is almost like a trump card, for if copies of a white gene exist, they mask all other colours. So even if a cat possesses the genetic make-up to be a tortie, it will still appear white due to the masking properties of this gene. Even if only one copy exists, the cat will still be white. This is the ace of spades when it comes to colours!

So, to recap

Black and red are dominant, so only one copy is necessary to see a physical trait presented.

Dilute is recessive. If only one copy exists, a cat is a dilute carrier. If two copies exist, black cats become blues, red cats become creams, and tortie cats become blue-creams.

Chocolate is a recessive gene which only modifies the black dominant. Any reds with two copies of the chocolate gene will remain red but are called red chocolates. Two copies of the gene on a black cat will make it a chocolate. One copy will make any cat a chocolate carrier.

The dilute modifies chocolate, if two copies exist, to become lilac.

White is the trump gene, masking all other colours. Any cat with even one copy of the white gene will appear as all white. Any cat with two copies of the gene will consistently produce offspring who are all white.





Ok, now that we have that sorted, we do, don’t we? It’s time to move on to patterns.

Patterns

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• Colourpoint is a recessive gene, so two copies are necessary for a cat to be pointed.




• Tabby is a dominant, so only one copy is necessary for tabby to become the pattern.




• Bicolour is also dominant. When one copy of the gene is present, a black cat becomes a black and white bicolour, a red cat a red and white bicolour, and a tortie a calico. Remember that with the influence of the dilute, you can also get blue and white bicolours and cream and white. If a cat has two copies of the gene, it will be mostly white and will be called a van.




• Shaded/smoke/shell: This too is dominant. One copy alone makes a black cat a black smoke, a red cat a red smoke and a tortie a tortie smoke. The smoke gene varies in its expression though, producing something called variable penetrance. This basically deals with the length of the hair shaft that the colour extends to. Smokes have a hair shaft that is mostly coloured, but with a white undercoat. When the colour extends 50% down the shaft, the cat is a shaded rather than a smoke. Some cats have colour just on the tips of the shaft, and these are called shells.




• There is one final pattern, silver/golden, but it is quite a complex one to explain given that it is a combination of three genes working together. If any of you wish to know more, please write me privately and I’ll direct you to an informative article to read.